Not only to estimate the direction and the strength of the solar radiation from the temperature data of a thermal image, but also to obtain an accurate estimation value of the atmospheric temperature without using an in-car sensor.

The existence of the solar radiation to a measuring object is decided based on the difference between the temperature data of the matrix elements of a thermal image measured by a matrix type sensor and a specific standard temperature, a solar radiation pattern of the thermal image made by discriminating the matrix elements by the existence or absence of the solar radiation is produced, the strength of the solar radiation to the measuring object is estimated depending on the difference between the average value of the temperature data of the matrix elements decided to exist the solar radiation of the solar radiation pattern, and a specific standard temperature, and the atmospheric temperature is estimated depending on the estimated strength of the solar radiation, and the background temperature of the thermal image.